Phosphorescent materials or phosphors have been employed for a number of years in the construction of cathode ray tubes having a display screen that is excited by an electron beam. When struck by the electron beam the phosphor material absorbs energy from the beam and subsequently re-emits the energy as electromagnetic radiation with wavelengths within the visible range.
Representative U.S. Patents in this general area include the following.
Wainer (U.S. Pat. No. 4,073,989) discloses a cadmium sulfide phosphor that may be deposited as ultra-fine particles on a glass or quartz base.
DiStefano (U.S. Pat. No. 4,082,889) discloses a cadmium sulfide (CdS) semiconductor material. Both the Wainer '989 and DiStefano '889 patents disclose the combination of CdS and ZnS in at least one embodiment. The luminescent layer as practiced by DiStefano comprises a polycrystalline thin film 40 of semiconductor material forming individual grains 36 oriented with random size, orientation, and configuration. The typical size of each grain 36 ranges from about 1 micron to about 100 microns. The film 40 is conductive in the lateral direction. A thin skin 35 of majority type dopant forms a layer which preferably covers all surfaces, whether they be the film surface 34, a grain boundary 38, or the interface 42 between the film 40 and the substrate 32.
Delahoy (U.S. Pat. No. 4,625,071) discloses cadmium sulfide semiconductor particles in a necklace configuration that range in size from 10 to about 3000 angstroms.
Mie (U.S. Pat. No. 4,081,716) discloses a fluorescent display element having an integrated semiconductor element 3 and a cadmium sulfide fluorescent display element.
Ellis (U.S. Pat. No. 4,559,116), Fraas et al. (U.S. Pat. No. 3,976,361) and Lee (U.S. Pat. No. 3,583,788) disclose cadmium sulfide semiconductors and/or graded cadmium sulfide crystals with and without other materials.
In an article entitled "Photochemistry of Semiconductor Colloids. 17. Strong luminescing CdS and CdS-Ag.sub.2 S Particles", Ber. Bunsenges Phys. Chem. 91, 88-94 (1987), L. Spanhel, H. Weller, A. Fojtik and A. Henglein, report the preparation of Q-CdS sols which fluoresce with quantum yields said to be close to 100%. Strong fluorescence is said to occur when defect sites, at which radiationless recombination takes place, are blocked.
In a paper entitled "Doped Nanocrystals of Semiconductors-A New Class of Luminescent Materials", The 1993 International Conference on Luminescence, TH1B-2, Aug. 9-13, 1993, Univ. of Conn., Storrs, Conn., R. N. Bhargava reports the incorporation of a luminescent center (Mn.sup.2+) in nanosize (30 .ANG.-70 .ANG.) particles of ZnS. The nanosize ZnS particles are said to show an increased energy bandgap due to quantum confinement, which is also said to effect the recombination kinetics of the Mn.sup.2+ luminescence. The 1.7 millisecond Mn.sup.2+ decay time in the bulk is reported to be shortened to about 3 nanoseconds after several hours of curing with UV radiation (300 nm).
In this regard reference is also made to a publication entitled "Optical Properties of Manganese-Doped Nanocrystals of ZnS", Physical Review Letters, Vol. 72, No. 3, pgs. 416-419, Jan 17, 1994, by R. N. Bhargava and D. Gallagher.
Both of these publications describe a method of doping ZnS nanocrystals with Mn.